The 15-metre sperm whale that wowed Perth swimmers with extra-close encounters at the weekend has sadly died.

Concerns grew for the sick and sunburned whale in the past 24 hours, after it got stuck on a sandbar at Rockingham Beach late on Monday, with local authorities warning it was likely to be euthanised.

In a later update on Tuesday, they confirmed the marine mammal had died.

Swan Coast district manager for Western Australia’s Department of Biodiversity, Conservation and Attractions, Mark Cugley said staff had been monitoring the whale but its conditioned had deteriorated as it lay on the sandbar. It died early on Tuesday.

The sad end to the story that has captured Perth and the internet followed a weekend encounter with the whale at Port Beach, near Fremantle. Vision of the whale, only a few metres from swimmers, went viral online.

Ori Weiser, who was among those to share the remarkable moment online, said it looked as though the whale was going to beach itself.

“He just came in for a quick beer,” Weiser said, responding to concerns about the whale’s health.

“Looked like was beaching himself but after a while, and support from all of us, swam back out happy.”

But, despite the happy encounter for the swimmers, one expert warned The New Daily that the out-of-character behaviour was concerning. Lawrence Chlebeck, a marine biologist for Humane Society International, said its approach to shore was very unusual for the species.

“Sperm whales live far off the coast. They dive very deep for squid and things like that, things that live way down in the dark depths, so this is very, very odd behaviour,” Chlebeck said.

On Tuesday, Cugley said the whale’s carcass showed no visible signs of external trauma. Its advanced age was a likely factor in its death.

“We will be looking at certainly doing some postmortem or necropsy analysis as well to understand anything more we can about the death of the whale and also about this species, given it’s quite unusual to have a sperm whale in this area of Perth,” he said.

Earlier, he’d said the whale was showing worrying signs.

“Yesterday, it was swimming in those tight circles, which isn’t normal behaviour, which … makes us concerned that not everything is well with the animal,” Cugley said.

“At times, it became grounded on shallow sandbars, and it would swim off again, then it would get grounded again. So it’s concerning, and that’s why we need to fully assess its health.

“It’s got a little bit of sunburn, and it’s a little bit sunken and depressed behind its melon (forehead), which isn’t a great sign of body condition.”

Cugley said the whale appeared to have lost body condition.

“It’s not got a lot of strength [and] it’s got a higher respiration rate, which suggests that it’s stressed,” he said.

Rockingham Beach has been closed, and a shark warning issued for Naval Memorial Park.

Cugley said authorities were working on a plan to remove the whale carcass from the beach.